In recent years, in the field of on-vehicle engines, particularly on-vehicle diesel engines, a multistage (two-stage) exhaust turbo-charging system has been adopted, in which a high-pressure stage turbocharger having a high-pressure turbine drivable by exhaust gas from the exhaust manifold of the engine and a low-pressure stage turbocharger having a low-pressure turbine drivable by exhaust gas from the high-pressure turbine are arranged sequentially in the flow path of the exhaust gas, and supply air for the engine pressurized by a low-pressure compressor of the low-pressure stage turbocharger is supplied via an air supply channel to a high-pressure compressor of the high-pressure stage turbocharger to be further pressurized by the high-pressure compressor and charged to the engine.
With an engine equipped with a multistage exhaust turbocharger as mentioned above, stable operation of the engine with high turbo-charging efficiency is attained, by increasing supper charge pressure in a low and middle speed operating range of the engine by performing two-stage supercharging by allowing both the high pressure stage and low-pressure stage turbocharger to operate, and by performing single-stage supercharging in a high speed operating range of the engine by allowing only the low-pressure stage turbocharger to operate by allowing the exhaust gas and supply air to bypass the high-pressure stage turbocharger.
This type of two-stage exhaust turbocharger is known for example from US 2003/0159442 A1, U.S. Pat. No. 6,378,308 B1, and JP 59-82526.
The two-stage exhaust turbocharger disclosed in US 2003/0159442 A1 (patent literature 1) includes a high-pressure stage exhaust turbocharger and a low-pressure stage exhaust turbocharger arranged compactly so that it is applicable to an on-vehicle engine mounted on a narrow engine room of a vehicle. The high-pressure stage and low-pressure stage turbochargers are arranged three dimensionally with the rotation axis of each turbocharger parallel to each other, and the exhaust gas outlet side of the turbine of the high pressure stage turbocharger is connected to the exhaust inlet side of the turbine of the low pressure stage turbocharger by an exhaust channel and the supply air outlet side of the compressor of the low pressure stage turbocharger is connected to supply air inlet side of the high pressure stage turbocharger by an air supply channel.
A comparatively large installation space is required in the engine room to install an engine equipped with a two-stage exhaust turbocharger of this type because it is inevitably increased in bulk as compared with a single-stage one.
It is demanded in the case of on-vehicle engine to reduce the bulk of the two-stage exhaust turbocharger as far as possible in order to mount an engine equipped with a two-stage exhaust turbocharger in the conventionally narrow engine room of the vehicle.
The two-stage exhaust turbocharger disclosed in the patent literature 1 is designed to meet the demand, however, there remains room for further improvement.
According to the art of the patent literature 1, the two-stage turbocharger is mounted compactly by arranging the high pressure stage and low pressure stage turbocharger three dimensionally with the rotation axis of each turbocharger parallel to each other, however, because the exhaust turbines are connected via the exhaust gas channel and the compressors are connected via air supply channel, two connecting operations are required, which result in increased assembling man-hours.
Particularly, as the high-pressure turbine and low-pressure turbine are connected by the exhaust gas channel in which exhaust gas of high temperature flows, the exhaust gas channel, gasket, and bolts are required, so number of parts increases.
Further, it is required to perform the connecting work such that perfect sealing of high pressure, high temperature exhaust gas is ensured, so assembling man-hours increase further.
In addition, as the connection of the turbochargers are performed by two connecting channels of the exhaust gas channel and air supply channel, volume occupied by the two-stage turbocharger increases, and mounting of an engine equipped with the two-stage exhaust turbocharger in a narrow engine room of a vehicle is not facilitated.